Foamed plastics are advantageously used in a wide field because of their functions, such as heat insulation, cushioning, and the like.
High extents of foaming of foamed plastics have been achieved by improvement of retention of cells during foaming by crosslinking or use of resins impregnated with a large quantity of a physical foaming agent. The former technique is preferred for obtaining foamed plastics having a uniform structure and excellent physical properties.
On the other hand, propylene-based resins are used in a very large field due to their excellent properties, such as moldability, transparency, electrical insulating properties, water resistance, chemical resistance, mechanical strength, and the like. However, in the propylene-based resins, cleavage of the main chain takes place by the action of a radical in preference to crosslinking reaction so that they are not easily crosslinked by radical crosslinking with peroxides or radiation, thus, failing to provide highly expanded foams. It has been proposed, as a countermeasure, to incorporate various crosslinking auxiliaries comprising polyfunctional compounds. However, this method encounters problems, such as non-uniformity in crosslinking, bleeding of low molecular weight substances, deterioration of weather resistance, reduction in various physical properties, and the like.
Many inventions have hitherto been disclosed with respect to copolymerization of .alpha.-olefins and non-conjugated dienes. Among them, the most relevant to the present invention are techniques described in British Pat. No. 1,268,149 and U.S. Pat. Nos. 3,933,769 and 3,991,262.
British Pat. No. 1,268,149 discloses use of a special titanium trichloride composition which is finely dispersed into a colloid as a transition metal component of a polymerization catalyst, thereby to provide a colloidal copolymer having an average particle size of from 0.02 to 0.5 .mu.m. With the purpose of producing fine copolymer particles for thin layer coating, the special titanium trichloride catalyst component is prepared by reducing titanium tetrachloride with an organic aluminum compound in the presence of a small amount of an .alpha.-olefin having 6 or more carbon atoms. Use of such a fine catalyst component leads to the production of markedly fine particles of the unsaturated copolymer. As a result, serious problems are likely to occur in the production on an industrial scale, such as an increase of viscosity in a polymerization vessel, difficulty in removal of polymerization heat, difficulty in recovery of the resulting unsaturated copolymer, and the like.
U.S. Pat. Nos. 3,933,769 and 3,991,262 relate to copolymers of .alpha.-olefins having from 4 to 12 carbon atoms and methyl-1,4-hexadiene, and are characterized in that the resulting copolymers are rubbery products. Therefore, these techniques are not applicable to production of resinous copolymers.
Seeking for a solution of problems associated with the conventional techniques, the inventors have previously proposed crosslinkable propylene-based unsaturated copolymer resins as disclosed in Japanese Patent Laid-Open Application Nos. 98534/82 and 210930/83. Nevertheless, these resins are not necessarily satisfactory as foamable molding materials. More specifically, when the propylene-based unsaturated copolymer resin is kneaded with a foaming agent, the melt viscosity is so high that heat is generated by shearing to raise the resin temperature above the decomposition temperature of the foaming agent. As a result, it is likely that a gas is evolved due to decomposition of the foaming agent during kneading to form large or small cells in the molded product, which ultimately leads to disadvantages, such as formation of large voids in the final foams, non-uniformity in cell size or low rates of expansion reached.